This invention relates to a reflective mask for exposure which is for use in manufacture of a semiconductor device and the like, a reflective mask blank as an original material of the reflective mask, and methods of producing the same.
In recent years, the advance of miniaturization of semiconductor devices awakens expectations of using EUV lithography as an exposure technique using extreme ultra violet (hereinafter abbreviated to EUV) light in the semiconductor industry. Herein, the EUV light represents light in a wavelength band of a soft X-ray region or a vacuum ultraviolet region and, specifically, light having a wavelength of approximately 0.2 to 100 nm. As a mask for use in the EUV lithography, a reflective mask for exposure is proposed, for example, in JP-B-H07-27198 (Patent Document 1).
The reflective mask of the type comprises a substrate, a multilayer reflective film formed on the substrate to reflect exposure light, and a patterned absorber film formed on the multilayer reflective film to absorb the exposure light. The exposure light incident to the reflective mask mounted to an exposure apparatus (pattern transfer apparatus) is absorbed in an area where the absorber film is present. On the other hand, in another area where the absorber film is not present, the exposure light is reflected by the multilayer reflective film to form an optical image which is transferred onto a semiconductor substrate through a reflective optical system.
As the above-mentioned multilayer reflective film, for example, which is adapted to reflect the EUV light having a wavelength of 13 to 14 nm, there is known a multilayer film comprising Mo and Si thin films each having a thickness of several nanometers and alternately laminated in about 40 to 60 cycles or periods, as shown in FIG. 3. In order to increase a reflectance of the multilayer reflective film, it is desired that the Mo film having a high refractive index is located at an uppermost layer. However, Mo at the uppermost layer is easily oxidized in contact with air. This results in decrease in reflectance. In view of the above, the Si film is located at the uppermost layer to serve as a protective film for preventing oxidation.
JP-A-2002-122981 (Patent Document 2) discloses a reflective mask comprising a multilayer reflective film composed of Mo films and Si films alternately laminated, an absorber pattern formed on the multilayer film, and a buffer layer of ruthenium (Ru) formed between the multilayer reflective film and the absorber pattern.